The present invention concerns a method to produce tesserae or slabs of glass mosaic, of the type containing an ornamental metal foil, for example gold, called golden mosaic. In the following description we shall talk essentially about gold foil or leaf, but it should be understood that the foils can be of silver, gold or silver alloys or other enhancing materials of a more or less precious nature.
The method according to the invention is concerned particularly with a process to make tesserae or slabs containing gold foil, wherein the processing steps are performed in sequence, substantially automatically and without the need for subsequent trimming or other manual interventions on the product.
The invention also concerns the plant for the automatic production of the tesserae of mosaic with gold leaf, and also the tesserae or slabs thus obtained.
One of the techniques used to make glass tesserae for mosaics aesthetically particular and valuable is to insert at least a layer of metal material, with thicknesses which can reach about 10÷30 μm, between two or more layers of transparent glass. This technique, which has very ancient roots, has evolved in the history of art, without substantially changing its method of production. In fact, in our own day, such types of tesserae continue to be mainly produced manually.
Metal foil, or the so-called gold leaf, is made to adhere to a previously cast glass plate resting on a metal plane suitably heated until it softens the glass; attention must be paid, however, that the melting point of the metal is not reached, which in the case of gold is around about 1050÷1080° C. In this step molten glass is also poured above the metal leaf so as to form a single three layer element: glass-metal-glass. Instead of casting molten glass, glass powder is added which, when it melts, forms a protective layer on the metal foil. The three-layer element is carefully pressed to ensure that the layers adhere properly.
Subsequently, the semi-worked product thus obtained is inserted into an annealing furnace to eliminate the surface tensions and then it is squared and cut manually by means of diamond tools.
This method is totally manual, which entails very long times and high production costs. Moreover, since this technique is difficult to do, satisfactory results are not always achieved even when glass-workers of considerable experience are supervising.
Another disadvantage is that, since they are made manually one by one, the tesserae often have an irregular shape, particularly along the edges, a different thickness and a shape not coherent with the geometrical shapes, such as the double-opposed inclined plane shape, of the non-enhanced tesserae produced with automated systems, with which they are laid on the walls or floors. This entails considerable problems during the forming step of sheets of mosaics which comprise some golden tesserae inserted into mosaics with non-enhanced tesserae, and entails the need for particular attention during laying.
EP 844.218 A, as an example of prior art, discloses a method for manufacturing gold slabs for mosaic comprising a step of applying a fluid suspension containing gold to a glass sheet of a first greater thickness, a step of heating the glass sheet and the suspension so as to obtain a film-like coating of the sheet, a step of protecting film-like coating with a layer of molten glass of a second lesser thickness, and a step of heating the layers so as to obtain a single sheet composed of the two layers of glass and the gold coating. This document, however, does not solve the shortcomings related to the edge shape of the mosaic gold slabs thus obtained. Moreover, the method is slow and requires a preparatory step to prepare the suspension containing gold and requires much skill and experience to obtain the desired thickness of the gold leaf from a dust suspension.
Furthermore, this document does not disclose how can be designed an apparatus for performing the method and directed to a mass, automatic and standard production of the gold slabs.
DE 10 85 304 B discloses a method for producing glass mosaic slabs containing inside a metallic layer, in which the inner metallic layer is covered by glass dust before the sandwich is placed in a furnace for the fusion. This document refers to a known technique in the art of the mosaic and entails all the shortcomings listed above.
The present Applicant has devised and embodied this invention to overcome the shortcomings of the state of the art and to obtain further advantages.